The invention relates to a mixture-compressing spark-ignition internal-combustion engine of the type having a cylinder head, which has a depression serving as a main combustion chamber and an auxiliary combustion chamber provided with a spark plug and connected to the main combustion chamber by an overflow bore, through which a part of the fuel/air mixture from the main combustion chamber is compressed by a piston and, after ignition has taken place, leaves at high pressure and velocity as an ignition jet with a jet direction aiming at a surface region of the piston crown.
An internal-combustion engine of the generic type is known, for example, from German Patent Specification No. 2,626,550. In this case, the auxiliary combustion chamber is arranged close to the upper crest region of the hemispherically shaped main combustion chamber. The auxiliary combustion chamber has, furthermore, an overflow bore of a relatively large cross-section. This is aligned with the main combustion chamber in such a way that the emerging ignition jet flows with a tangential flow component into the main combustion chamber and consequently sets the compressed fuel/air mixture in motion. Due to the described position of the overflow opening in the main combustion chamber, the ignition of the mixture begins in the upper section of the main combustion chamber, from which subsequently the flame front spreads in the direction of the piston. During this process, unburned fuel/air mixture is compressed into the fire land gap of the piston, which is the main cause of increased residual hydrocarbons in the exhaust.
An object of the invention is to redesign a known internal-combustion engine with a main combustion chamber and an auxiliary combustion chamber in such a way that a complete burning of the fuel/air mixture with a high mass fraction and greatly reduced fractions of pollutants, in particular of unburned hydrocarbons, is achieved.
This object is achieved according to preferred embodiments of the invention by providing an arrangement wherein the overflow bore is aligned concentrically with respect to the longitudinal center axis of the cylinder and the surface region of the piston crown is designed as a level impact area lying opposite the overflow bore and running orthogonally with respect to the jet direction of the ignition jet.
It is achieved by the invention that the ignition jet leaving the overflow bore at high pressure and velocity strikes the impact area on the piston crown after a shortest possible path. It is thereby ensured by a relatively short jet path that the ignition jet is not broken up by mixture movements in the main combustion chamber and that, in any event, initially only a proportion of mixture lying in the peripheral region of the ignition jet is ignited. Due to the orthogonal position of the impact area with respect to the direction of the ignition jet, the latter is fanned out evenly in radial direction after striking the impact area. The tongues of fire shooting away as a result in radial direction over the piston crown produce a surface fire above the piston crown, which, after reaching the cylinder wall, is deflected into the depression of the cylinder head and partially into the fire land gap. The ignition of the fuel/air mixture in the main combustion chamber thus begins by the surface fire or the fire veil above the piston crown. The flame front emerging from this spreads in this case in the direction of the depression in the cylinder head crown. The unburned fuel/air mixture ahead of the flame fron is consequently compressed under increasing pressure in the direction of the depression. With the invention, thus a combustion sequence which has the particular advantages mentioned below is achieved:
Due to the fire veil, the fire land gap is isolated from the unburned mixture and, together with the movement of the flame front, it is prevented that unburned mixture can be forced into the fire land gap. As a result, a considerable reduction in the unburned hydrocarbons in the exhaust is achieved. PA1 Owing to the expansive ignition of the fuel/air mixture, the combustion takes place with a high energy conversion rate. As this is associated with an increase in the constant-volume combustion, it results in an improvement in the process efficiency. PA1 Due to the high energy conversion rate, and the concomitant high speed at which the mixture completely burns, the occurrence of an entrained combustion is reduced. PA1 Due to the expansive ignition of the mixture, good preconditions for the use of much leaner mixtures without appreciable disturbances in the combustion sequence are provided.
According to preferred embodiments of the invention an arrangement is provided wherein a shielding of the ignition jet from the mixture compressed in the main combustion chamber is provided, as a result of which an ignition of parts of the mixture in the vicinity of the ignition jet is prevented to a great extent.
In certain preferred embodiments, arrangements are provided wherein in an upper dead center position of the piston, the impact area has a distance from the mouth of the overflow bore of approximately 4 mm to 10 mm. In certain preferred embodiments, arrangements are provided wherein the volume V.sub.HB of the auxiliary combustion chamber has approximately 0.01 to 0.03 of the compression volume V.sub.C of the main combustion chamber. These dimensional ranges have the effect of ensuring that the ignition jet strikes the impact area on the piston crown with both the energy required for fast burning of the mixture and with a desired high velocity.
With a design of the main combustion chamber according to certain preferred embodiments wherein the depression in the cylinder head crown has the shape of an inverted omega as seen in cross-section, it is achieved that the flame front emerging from the piston crown advances as equidistantly as possible in the direction of the wall limitation on the cylinder head side of the main combustion chamber.
In order that the ignition jet fanned out over the piston crown reaches the cylinder walls in the shortest possible time, it is advantageously provided in certain preferred embodiments that the piston crown is provided with an omega-shaped hollow as seen in cross-section, the elevation of which hollow in the region of the longitudinal center axis of the cylinder is flattened off as a level impact area. In other preferred embodiments the piston crown has a calotte-shaped hollow. In other preferred embodiments the piston crown has an overall level surface.
Finally, it is advantageous for the progression of the ignition jet according to especially preferred embodiments if the overflow bore has a diameter of 1 to 3 mm.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.